The present invention relates to a crystal puller for growing single crystal semiconductor material, and more particularly to a heat shield for use in such crystal pullers.
Single crystal semiconductor material, which is the starting material for fabricating many electronic components, is commonly prepared using the Czochralski ("Cz") method. In this method, polycrystalline semiconductor source material such as polycrystalline silicon ("polysilicon") is melted in a crucible. Then a seed crystal is lowered into the molten material and slowly raised to grow a single crystal ingot. As the ingot is grown, an upper end cone is formed by decreasing the pull rate and/or the melt temperature, thereby enlarging the ingot diameter, until a target diameter is reached. Once the target diameter is reached, the cylindrical main body of the ingot is formed by controlling the pull rate and the melt temperature to compensate for the decreasing melt level. Near the end of the growth process but before the crucible becomes empty, the ingot diameter is reduced to form a lower end cone which is separated from the melt to produce a finished ingot of semiconductor material.
In order to control the diameter of the ingot, the actual diameter of the ingot must be measured throughout the pulling process. A camera is mounted above the crucible for automatically determining the diameter of ingots as they are pulled. The camera measures the positions of at least three points on a meniscus formed between the ingot and an upper surface of the molten material. The camera is able to distinguish the meniscus from the surrounding material because the meniscus appears brighter than the surrounding material. The diameter of the ingot can be calculated from these points using geometric formulas which are well known by those skilled in the art.
Although the conventional Cz method is satisfactory for growing single crystal semiconductor materials for use in a wide variety of applications, further improvement in the quality of semiconductor material is desirable. For instance, as semiconductor manufacturers reduce the width of integrated circuit lines formed on semiconductors, the presence of defects in the material becomes of greater concern. Defects in single crystal semiconductor materials form as the crystals solidify and cool in the crystal puller. Such defects arise, in part, because of the presence of an excess (i.e., a concentration above the solubility limit) of intrinsic point defects known as vacancies and self-interstitials. Vacancies, as their name suggests, are caused by the absence or "vacancy" of one or more atoms in the crystal lattice. Self-interstitials are produced by the presence of one or more extra atoms in the lattice. Both kinds of defects adversely affect the quality of the semiconductor material.
Ingots are typically grown with an excess of one or the other type of intrinsic point defect, i.e., either crystal lattice vacancies or self-interstitials. It is understood that the type and initial concentration of these point defects in the ingots, which become fixed as they solidify, are controlled by the ratio of the growth velocity (i.e., the pull rate) (v) to the instantaneous axial temperature gradient in the ingot at the time of solidification (G.sub.o). When the value of this ratio (v/G.sub.o) exceeds a critical value, the concentration of vacancies increases. Likewise, when the value of v/G.sub.o falls below the critical value, the concentration of self-interstitials increases. Although neither type of defect is desirable, growth regimes which produce more vacancies are preferred, in general, by the semiconductor industry. The density of intrinsic point defects may be reduced by controlling v/G.sub.o to grow a crystal lattice in which crystal lattice vacancies are the dominant intrinsic point defect, and by reducing the nucleation rate of agglomerated defects by altering (usually, by lowering) the thermal gradient G.sub.o in the silicon ingot when its temperature is within a range of about 1150.degree. C. to 1050.degree. C. during the crystal pulling process.
In order to produce vacancy rich ingots, and avoid the presence of a radial vacancy/self-interstitial boundary ring in the ingots, v/G.sub.o is controlled to be as high as possible. One way to increase this ratio is to increase the pull rate (i.e., growth velocity, v) of the ingot. However, the pull rate also affects other parameters, such as the ingot diameter. Thus, the amount the pull rate may be increased is limited.
The other way to increase the ratio is to reduce the thermal gradient G.sub.o in the ingot. To this end, a heat shield may be positioned within the crucible above the melt surface for conserving heat at the interface between the ingot and the molten material to prevent heat loss from the melt surface. In this way, the instantaneous axial thermal gradient G.sub.o at the interface is reduced, which increases the ratio v/G.sub.o. These heat shields generally include a central opening through which the ingot is pulled as it is grown from the melt. In the past, the central opening was made large enough to permit the camera which determines the ingot diameter to view the points on the meniscus through the opening. Otherwise, the shield would have obstructed the view of the camera. Since the central opening was relatively large to enable the camera to view the points, a considerable amount of heat escaped upward past the shield, thereby significantly reducing the effectiveness of the heat shield.